1. Field of the Invention
This invention relates to the field of optical transmission. More particularly, this invention relates to a method for transmitting energy and information optically through biological tissue.
2. Art Background
A common method of coupling to a nervous system is to thread individual nerve fibers through ring shaped electrodes made by etching a silicon chip. This interface method is part of a class of interfaces called "chronic" neural interfaces. A chronic neural interface is a way of tapping into, at this point in the evolution of the technology, peripheral nerves in mammals. This technique has been demonstrated at Stanford University.
The Stanford device, in particular, involves integrated circuits that have had holes etched through them. A nerve is cut and a little holder for the nerve is placed across the cut end of the nerve. The nerve regenerates through the holes in the chip, providing small groups of axons in each hole. The holes have electrodes around them that allow one to drive and sense the neurons, so neural activity can be monitored and controlled. Alternative arrangements using metal and indium-tin-oxide electrodes on the surface of chips have also been demonstrated.
Alternate chronic neural interfaces are being developed elsewhere (e.g. the Veteran's Administration and the University of Michigan, Ann Arbor). Although still in the pure research stage, another such method to attach nerve cells to the chip is to induce them to grow to the electrode sites. Instead of cutting the nerves and threading them through holes, bioengineers are beginning to provide tools that allow one, with the right kind of implant, to actually request that a nerve bundle grow a tap over to a chip. This technique is much less invasive than severing a nerve.
Until recently, the dogma was that once neurons were formed in the central nervous system of mammals, no additional neurons could be formed. Epithelial growth factor has been found to stimulate division of neurons in the central nervous system of a mouse.
Using electrically sensitive polymers as a controlled long term drug delivery mechanism provides a possible way to cause a nerve bundle to grow a tap over to a chip. Small amounts of the polymer are placed at each electrode site, and the polymer is loaded with the correct mixture of cell growth factors. Stimulation of the polymer by the electrodes on the chip results in a gradient of growth factors. Under proper conditions, nerve cells will grow up the gradient, and attach themselves to the electrode site.
All electrode sites are made bi-directional, that is, each site is equipped with both drive and receive electronics. Therefore, it is possible to provide a control signal to the polymer, and to monitor for the presence of nerve cells. If each electrode site is individually controllable, one can stop driving a particular site when a nerve cell has arrived, and then drive a neighbor site to attract the next nerve cell to grow towards the chip.
The intention behind developing chronic neural interfaces is (1) to help provide a better understanding of the nervous system; and (2) to control prosthetics. The latter application helps people who have had amputations, as well as people with other problems, such as paraplegics or quadriplegics. Examples of possible prosthetic applications of this technique include hearing and vision replacement, and "jumpering" across a severed nerve to restore some amount of motor control and sensation. Thus, it is possible to reach into what is left of a damaged peripheral nerve, induce it to grow through the holes in a chip, and then extract neural signals to provide controls for an artificial arm or an artificial leg. It is also possible to provide other links. For example, it is possible to provide a link into a computer so that the computer could be controlled to provide services for a person who is paralyzed.
Furthermore, in general, it has been very difficult clinically to penetrate the skin in a long term fashion with wires or with other connectors. For example, in kidney dialysis, a vein and artery are shunted, usually in the forearm. These shunts require constant attention to prevent infections.
Therefore, for chronic neural interfaces, it is desirable to bring electrical signals in and out through the skin, but it is preferable to achieve this without using wires to penetrate the skin. Currently, the most common practice is to have signals brought in and brought out through the skin using radio transmission and reception.
It is also common to reprogram a pacemaker which has been embedded subdermally. Pacemakers are currently reprogrammed by tapping the chest with a magnet that operates a reed switch inside the pacemaker. With current technology, the reprogramming is performed at a painstakingly low data rate, in the order of bits per second. The programmer must tap with the magnet and then examine an electrocardiogram readout to determine whether the programming was performed correctly.
Simply providing power to a subdermal device is also important. For example, pacemakers currently have to be surgically removed before their batteries expire and new ones inserted. Generally, one does not want to perform surgery if it can be avoided. Therefore, it is desirable to transmit power to a subdermal site and thereby power the system.
Another current problem encountered by chronic neural interfaces is that of providing power to the subdermal chips. If there is subdermal processing circuitry that is reading, or driving the nerves, there must be electrical power for the circuitry to operate. Electrical power is most often provided by split transformer, where one half of the core of the transformer is placed inside the skin and the other half of the core is placed outside the skin. These transformers tend to be bulky objects and it is difficult to move much power through them. For example, cochlear implants are implants that are placed on the cochlea of an ear to jumper damage to either the cochlea, itself, or to repair damage that has been done to the mechanical structure of the ear and thereby bring sound to the cochlea. The cochlea is a very tight place to work, and one would prefer not to be fettered by the bulk of a split transformer device.
Previous attempts to use light through the skin as the medium of bringing both power and signal in and bringing signal out have proved to be unsatisfactory. Light as a medium has not had much utility because, for people, it is desirable to place the receiving chip a little bit below the surface of the skin so that there is not an obvious lump in the person's arm (or wherever the implant is placed). However, to provide light, which is bright enough to shine through the skin, to something that is located significantly beneath the skin requires a bright light source. Alternately, one must place the external module close to the skin and align it carefully with the implant. People tend not to be very good at the performing the required alignment.
Recently, it has become quite inexpensive to generate a relatively large amount of laser light with a laser diode. A continuous wave (CW) laser diode can be used to illuminate an array of photo detectors that are connected electrically in series. A CW laser is a laser that emits energy in an uninterrupted stream rather than in spurts. These laser diodes are readily available at wavelengths between 670 nm and 1.55 .mu.m. Wavelengths of 1.3 .mu.m and 1.55 .mu.m are common wavelengths that have been picked because of the utility in matching the minimum attenuation and dispersion points in optical fibers for telecommunications. If the emission wavelength of the laser is tuned to a high quantum efficiency frequency for the detector, efficiencies in excess of 50% may be realized.
The bandwidth attainable with current generation laser diodes and detectors is approximately one gigabit per second which is roughly comparable to the total bandwidth of a single human optic nerve.
A monolithic series stacked gallium arsenide (GaAs) photodiode array suitable for power reception has been demonstrated by Varian Associates, Palo Alto, Calif., for use in "power down the fiber" telephone applications. Furthermore, manufacturers, such as Laser Diode Laboratories, make a sugar cube sized laser diode, which is actually intended for soldering, that emits 10 watts of infrared light. Moreover, the current state of the art for surface emitting laser diodes is a threshold current of about 100 microamps at a forward voltage of about 1.5 volts. This represents a power consumption level of approximately 150 microwatts. Power consumption at that level is negligible when compared to the 50 to 100 milliwatts that can be delivered by an optical power transmission system.
As will be disclosed, the present invention provides a method for providing power and a high speed bi-directional data link through skin tissue without requiring an electrical shunt through the skin. Signals and power are carried as infrared light.